Reflective current and magnetic sensors based on optical sensing with integrated temperature sensing

ABSTRACT

Optical techniques and sensor devices for sensing or measuring electric currents and/or temperature based on photonic sensing techniques in optical reflection modes by using optical dielectric materials exhibiting Faraday effects are provided in various configurations. The disclosed optical sensing technology uses light to carry and transmit the current or temperature information obtained at the sensing location to a remote base station and this optical transmission allows remote sensing in various applications and provide a built-in temperature calibration mechanism to enhance the measurement accuracy in a range of different temperature conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent document claims the benefit of priority of U.S. ProvisionalPatent Application No. 62/295,894, filed on Feb. 16, 2016. The entirecontent of the before-mentioned patent application is incorporated byreference as part of the disclosure of this document.

TECHNICAL FIELD

This patent document relates to techniques and devices for sensing anelectric current and temperature.

BACKGROUND

An electric current is an electrical signal due to flow of charges in anelectrically conductive path such as a metal. Given the electricalnature of the electric currents, techniques and devices for sensing ormeasuring an electric current have been largely based on electroniccircuits. Electronic circuits usually require electrical power and canbe adversely affected by electromagnetic interference (EMI). Variouselectronic circuits for sensing currents need to be located at thelocations where the currents are measured and this may impose practicallimitations in various applications.

SUMMARY

This patent document discloses techniques and devices for sensing ormeasuring electric currents and temperature based on detecting reflectedlight from a photonic sensor head deployed at a location where a currentor temperature is measured. In the disclosed optical sensing technology,the sensing media are optical dielectric materials exhibiting Faradayeffects and therefore provide immunity to EMI. The disclosed opticalsensing technology uses light to carry and transmit the current ortemperature information obtained at the sensing location to a remotebase station and this optical transmission allows a long distance remotesensing. In addition, the optical processing of the detected signalsprovides additional advantages that are missing or difficult to achievein other sensing techniques that are entirely based on electroniccircuitry.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a first embodiment of the reflective current sensor. Aremote light source with low degree of polarization (DOP) with a pigtailof either single mode (SM) fiber or multimode fiber is used.

FIG. 2 shows an example of a temperature sensor head based on theoptical sensor head design in FIG. 1 by removing the Faraday sensingmaterial in FIG. 1 so that the magnetically shielded permanent Faradayrotator is used as a temperature sensing material.

FIG. 3 illustrates an example of a magnetic/current sensor head packagedwith a temperature sensor head in the optical reflective sensor headconfigurations shown in FIGS. 1 and 2.

FIG. 4 illustrates another example of a magnetic/current sensor headpackaged with a temperature sensor head b using a common low DOP lightsource and an optical coupler (Coupler 1) that splits the output probelight beam from the light source into two different probe beams to bedirected to the temperature sensor head and the current/magnetic sensorhead via two fiber lines, respectively.

FIG. 5 shows another example of a magnetic/current sensor head based onthe design in FIG. 1 with additional features, where a Wollaston prismis used to replace the polarizer in FIG. 1 and a third optical detectoris used.

FIG. 6 shows an example of a temperature sensor head based on the sensorhead design in FIG. 5 in which the Faraday sensing material in FIG. 5 isremoved and the magnetically shielded permanent Faraday rotator is usedfor sensing the temperature.

FIGS. 7-11 show implementation examples of optical sensor heads useprobe light at two different optical wavelengths for current sensing andtemperature sensing based on wavelength division multiplexing.

FIG. 7 shows a first embodiment of a WDM (wavelength divisionmultiplexing) based magnetic/current sensor with temperature monitoringand compensation.

FIG. 8 illustrates a second embodiment a WDM based magnetic/currentsensor with temperature monitoring and compensation.

FIG. 9 illustrates a third embodiment of WDM based magnetic/currentsensor with temperature monitoring and compensation.

FIG. 10 shows a fourth embodiment of a WDM based magnetic/current sensorwith temperature monitoring and compensation by using another way toseparate and monitor light from different WDM channels at λ1 and λ2.

FIG. 11 illustrates a fifth embodiment of a WDM based magnetic/currentsensor with temperature monitoring and compensation.

FIG. 12 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 1 andother examples described in this document.

FIG. 13 illustrates an embodiment of a reflective magnetic sensor basedon using a length of fiber to replace the Faraday sensing material inFIG. 5 with two fiber lines.

FIG. 14 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 7.

FIG. 15 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 8.

FIG. 16 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 9.

FIG. 17 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 10.

FIG. 18 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 11.

FIG. 19 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 11.

FIG. 20 illustrates another embodiment of a reflective magnetic sensorusing a length of fiber as the sensing medium as an example of anothersensor design.

DETAILED DESCRIPTION

Sensing of an electric current based on a fiber optic current sensor isattractive for monitoring, control, and protection of substations andpower distribution systems in smart grid. Comparing with some existingcurrent sensors, they have the advantage of being able to separate thesensor head that senses the current to be measured and electronicprocessing unit for processing a signal that contains the currentinformation at different locations, and can use the optical sensingnature of such technology to eliminate any electrical power andelectronic circuitry at the sensor head, making the sensor head immuneto electromagnetic interferences or damages triggered by strongelectromagnetic fields or interferences and safe for high voltageapplications. Implementations of fiber optic current sensors can beconfigured to achieve other advantages, including small size, lightweight, low power consumption, immunity to current saturation or others.Some implementations of optical current sensors based on Faraday effectsin glasses or crystals have the advantages of compact size, lightweight, low cost, and easy installation e.g., being able to be directlymounted on a current conducting cable even with a live current flow, andcan significantly reduce the installation cost. Because of the low costand small size, such optical current sensors can be applied to low andmedium voltage applications for transformer monitoring and other currentsensing applications.

Accuracy, dynamic range, and environmental stability are the mainperformance indicators for the fiber optic current sensors. An effectiveway to increase the dynamic range is to increase the magnetic fielddetection sensitivity, requiring significantly reducing the noise inoptoelectronics detection circuitry. The temperature dependence of theVerdet constants of the Faraday glasses or crystals poses challenges forthe for the measurement accuracy and need to be addressed in the sensorsystems design. The temperature effect on the optical signal derivedfrom the sensor head can impact the environmentally stable operation ofthe sensor, and thus it is advantageous to provide a mechanism forcompensating for the temperature effect. In addition, interferences ofthe magnetic fields from the neighboring current carrying conductors canalso affect the current detection accuracy, and thus should be reducedor minimized.

In the disclosed technology in this patent document, optical sensing canbe used to remotely measure electric currents or temperature bydirecting probe light from a base station via an optical fiber to anoptical sensor head deployed at a target location to use the probe lightto obtain and carry the information of the current or temperature to bemeasured and redirecting the probe light from the optical sensor headback to a base station for processing. Certain technical features inconnection with the technology disclosed in this patent document arerelated to or described in U.S. Patent Publication No. US 2015/0097551A1 on Apr. 9, 2015 of U.S. patent application Ser. No. 14/509,015entitled “FARADAY CURRENT AND TEMPERATURE SENSORS” and filed on Oct. 7,2014 based on U.S. Provisional Application No. 61/887,897 filed Oct. 7,2013. Both applications are incorporated by reference as part of thispatent document.

Fiber optic current sensor is attractive for various applications,including, e.g., the monitoring, control, and protection of substationsand power distribution systems in smart grid and in aluminum productionusing electrolysis process. Comparing with traditional current sensors,they have the advantage of being able to separate the sensor head andelectronic processing unit in different locations, and therefore do notrequire any electrical power at the sensor head, making it safe for highvoltage applications. In addition, they are generally more accurate thantheir electrical counterparts, especially when performing measurementsover wide temperature ranges. Other advantages include small size, lightweight, immune to electromagnetic interferences, low power consumption,and no current saturation. Accuracy, dynamic range, and environmentalstability are the main performance indicators for the fiber opticcurrent sensors. An effective way to increase the dynamic range is toincrease the magnetic field detection sensitivity, requiringsignificantly reduction in the noise in optoelectronics detectioncircuitry. The temperature dependence of the Verdet constants of theFaraday glasses or crystals poses challenges for the for the measurementaccuracy and must be overcome in the sensor systems design. In addition,the temperature effect on an optical signal derived from the sensor headcan affect the measurements and thus should be calibrated or compensatedfor achieving environmentally stable operation of the sensor.

Compact sensor designs may use a sensor head that is polarizationsensitive and the output powers can fluctuate with the polarization ofthe input light changes. Such polarization sensitivity can be reduced byusing a light source with a low degree of polarization (DOP), such as anAmplified spontaneous emission (ASE) source, in optical configurationsillustrated in this document. The reflective designs shown in theexamples in this document can be used to simplify the overall opticalpackaging of such optical sensors to provide improved sensingperformance.

For example, some implementations of the disclosed reflective designs donot impose a requirement on the polarization orientation of the inputoptical device with respect to the input light with low DOP. This aspectof the designs reduces the sensitivity of the optical sensor head to theoptical polarization and is beneficial. For another example, a polarizerplaced after the Faraday sensing material may be removed in variousdesigns. This is beneficial in part due to the reduced complexity anddevice components in the optical sensors. In addition, temperaturesensing can be implemented based on current sensor designs and can beused for compensating temperature sensitivity of the Faraday materialused in the current sensor head. Reflective designs using optical fiberas the sensing material are also provided in this document. Somedisclosed designs are configured to integrate the current sensor withthe temperature sensor for achieving high environmental stability withlow cost.

In the examples described in this document, the current sensor head isshown as an all optical current sensor head formed by optical componentswithout local electronic circuits or components. In this regard, thesensor head does not include any optical detector that converts lightinto electrical signals or any detector circuitry for electrical signalprocessing. The sensor head directs light carrying the measurementinformation of the current to the current sensor base station via afiber link. In this context, the sensor head is an all-optical currentsensor head and is placed by or at a current-carrying conductor to sensethe current based on the optical polarization rotation caused by themagnetic field associated with the current in the Faraday materialinside the all-optical current sensor. When the current to be measuredis perpendicular to the light propagation path of the Faraday rotator,the magnetic field produced by the current is along the lightpropagation path of the Faraday rotator and thus exerts most influenceto the polarization rotation of the light. The probe light from thebroad bandwidth light source is directed to the current sensor head anda returned light beam is directed back from the current sensor head tothe photodiode (PD) detection unit within the current sensor basestation via fiber and is detected to extract information on the electriccurrent sensed by the current sensor head. The low DOP property of theprobe light is to assure the optical power stability passing through thesensor head if single mode fiber (SMF) or multimode fiber (MMF) is usedto deliver light into the sensor head located remotely at the currentsensing site. If a more expensive polarization maintaining (PM) fiber isused, a light source with high DOP can be used and the polarization axisof the probe light is aligned with the slow (or fast) axis of the PMfiber.

FIG. 1 shows a first embodiment of the reflective current sensor. Aremote light source with low degree of polarization (DOP) with a pigtailof either single mode (SM) fiber or multimode fiber is used. If the DOPof the light source is not sufficiently low, a depolarizer can be usedto reduce the DOP. The light with low DOP is brought to the optical headlocated at a place with a magnetic field or current conductor to besensed. The low DOP light is first collimated, then passes or transmitsthrough an optical polarizer chip/device to be repolarized before itpasses through a permanent Faraday rotator with a nominal 22.5 degreerotation angle. This permanent Faraday rotator is magnetically shieldedfrom influence of any external magnetic field, preventing the influenceof the magnetic field from a current such as the current to be sensed.In addition to the magnetically shielded permanent Faraday rotator, thisdesign includes a sensing Faraday material that is not magneticallyshielded and is exposed to a magnetic field from a current to bemeasured. This magnetic field from the current to be measured causes arotation in polarization of the sensing Faraday material. Thispolarization rotation caused by the current is used to measure thecurrent. The polarization rotation angle produced by the sensing Faradaymaterial varies with the temperature of the material and this dependenceof the rotation with the temperature, when the material is free of anexternal magnetic field along the light propagation direction, can beused to measure the temperature at the material. The polarized the lightthen goes through the sensing Faraday material before it is reflectedback by an end mirror towards the collimator at the input/output port ofthe optical sensor head. Under this optical reflective design, thereflected light in the optical sensor head passes through themagnetically shielded permanent Faraday rotator twice so that itspolarization rotation after the second passing is 45 degrees from thepassing axis of the polarizer when the external magnetic field to besensed is absent. This 45-degree rotated polarization in the reflectedoptical output from the optical sensor head has a certain optical powerlevel. When the polarization rotation is different from the 45 degrees,the deviation in the polarization rotation from the 45-degreepolarization can be reflected in the measured optical power of thereflected optical output from the optical sensor head. When thisdeviation in the polarization rotation from the 45-degree polarizationis caused by the external magnetic field, the measurement of the opticalpower deviation in the optical power passing through the polarizer andentering the fiber can be used to determine the magnitude of theexternal magnetic field and thus the current that produces the externalmagnetic field. The detected photocurrent I₁ at the photodetector PD1is:I ₁ =I ₀(1+sin 2θ)  (1)whereθ=α₁(T)M+α ₂(T−T ₀)  (2)is the polarization rotation angle caused by the magnetic field and thetemperature to be sensed, α₁(T) is the proportional constant relatingthe magnetic field M to the rotation angle and is temperature dependent,α₂ is the is proportional constant relating the rotation angle θ to thetemperature, and T₀ is the temperature at which 0=0 in the absent of themagnetic field. For an alternating magnetic field, such as thatgenerated by an alternating current (AC), the rotation angle, which isproportional to the magnetic current or current, can be obtained byseparating the AC and DC component of Eq. (1) and taking their ratio:θ=α₁(T)M+α ₂(T−T ₀)=I ₁/2I ₀,  (3)For a DC current or magnetic field, the signal from the monitoring PD2is required to eliminate any optical power drift of the light sourceover time.

The current sensor example in FIG. 1 can be implemented to provide acurrent sensor based on optical sensing. Such a device can include alight source that produces probe light; a fiber line having a firstfiber line terminal coupled to receive the probe light from the lightsource and to direct the received probe light to a second fiber lineterminal of the fiber line; and an optical current sensor head coupledto the second fiber line terminal of the fiber line to receive the probelight. The sensor head can be configured to include an input opticalpolarizer to filter the probe light to produce a polarized input beam, afirst Faraday material that is magnetically shielded from an influenceof any external magnetic field and located along an optical path of thepolarized input beam, a second Faraday material that is not magneticallyshielded and placed in an optical path of the polarized input beam tosense a magnetic field produced by a current to be measured, and anoptical reflector downstream from the first and second Faraday materialsalong an optical path of the polarized input beam to reflect thepolarized input beam back to the first and second Faraday materials andthe input optical polarizer to return to the fiber line. The sensordevice can further include an optical detection unit including anoptical detector coupled to the fiber line to receive at least a portionof the reflected light in the fiber line from the optical current sensorhead that carries information of the current to be measured; and ameasurement module that receives an detector output from the opticaldetector to obtain information of the current. In some implementation,the measurement module can include an amplifier circuit to amplify thedetector output from the optical detector, an analog to digitalconverter to convert an output of the amplifier circuit into a digitalsignal, and a microprocessor to process the digital signal representinginformation of the current.

The disclosed technology can be used to construct a temperature sensorbased on optical sensing. Such a device can include a light source thatproduces probe light; a fiber line having a first fiber line terminalcoupled to receive the probe light from the light source and to directthe received probe light to a second fiber line terminal of the fiberline; an optical temperature sensor head coupled to the second fiberline terminal of the fiber line to receive the probe light. This headcan be configured to include an input optical polarizer to filter theprobe light to produce a polarized input beam, a Faraday material thatis magnetically shielded from an influence of any external magneticfield and located along an optical path of the polarized input beam andvaries a Faraday rotation angle in response to a change in temperatureat the Faraday material, and an optical reflector downstream from theFaraday material along an optical path of the polarized input beam toreflect the polarized input beam back to the Faraday material and theinput optical polarizer to return to the fiber line. The device furtherincludes an optical detector coupled to the fiber line to receive atleast a portion of the reflected light in the fiber line from theoptical temperature sensor head that carries information of thetemperature to be measured; and a measurement module that receives andetector output from the optical detector to obtain information of thetemperature.

In FIG. 1, two optical detectors PD1 and PD2 are provided where PD1 isoptional and may be used to monitor the power level of the probe beamgoing into the current/magnetic sensor head and PD2 is used to monitorthe power level of the reflected probe beam from the current/magneticsensor head.

FIG. 2 shows an example of a temperature sensor head, in which theFaraday sensing material in FIG. 1 is removed. The permanent Faradayrotator is used to sense a rotation change caused by the temperature inFIG. 2 and may be magnetically shielded to reduce a undesired influenceto its rotation by an external magnetic field. When operated in alocation where the external magnetic influence is insignificant or knownto be a certain constant level, the magnetic shielding may not be neededsince the rotation caused by the presence of the known magnetic fieldcan be calibrated in measuring the local temperature. The temperature Tcan be obtained as from Eq. (3) as:T=T ₀+(I ₁ −βI ₂)/2α₂  (4)where I₁ is the photocurrent in PD1 and I₂βI₀ is the photocurrentdetected by PD2.

FIG. 3 illustrates an example of a magnetic/current sensor head packagedwith a temperature sensor head in the optical reflective sensor headconfigurations shown in FIGS. 1 and 2 based on disclosed technology. Thetemperature obtained from the temperature head can be used to calibrateout the temperature dependence of the current sensor head. In thisexample, two different low DOP light sources are used for respectivelyproducing the probe light beams to the temperature sensor head and tothe current/magnetic sensor head via two separate fiber lines. Twooptical couplers are coupled in the two fiber lines, respectively, forcoupling light to optical detectors to measure the current and thetemperature t the sensor heads. For the temperature sensor operation,two optical detectors PD1 and PD2 are provided where PD1 is used tomonitor the power level of the probe beam going into the temperaturesensor head and PD2 is used to monitor the power level of the reflectedprobe beam from the temperature sensor head. For the current/magneticsensor operation, two optical detectors PD3 and PD4 are provided wherePD3 is optional and may be used to monitor the power level of the probebeam going into the current/magnetic sensor head and PD4 is used tomonitor the power level of the reflected probe beam from thecurrent/magnetic sensor head.

FIG. 4 illustrates another example of a magnetic/current sensor headpackaged with a temperature sensor head b using a common low DOP lightsource and an optical coupler (Coupler 1) that splits the output probelight beam from the light source into two different probe beams to bedirected to the temperature sensor head and the current/magnetic sensorhead via two fiber lines, respectively. The reflected probe beams fromthe two sensor heads are then coupled out of their respective fiberlines by optical couplers 2 and 3 for optical sensing operations. Forthe temperature sensor operation, two optical detectors PD1 and PD2 areprovided where PD1 is used to monitor the power level of the probe beamgoing into the temperature sensor head and PD2 is used to monitor thepower level of the reflected probe beam from the temperature sensorhead. For the current/magnetic sensor operation, two optical detectorsPD3 and PD4 are provided where PD3 is optional and may be used tomonitor the power level of the probe beam going into thecurrent/magnetic sensor head and PD4 is used to monitor the power levelof the reflected probe beam from the current/magnetic sensor head. Thetemperature obtained from the temperature head can be used to calibrateout the temperature dependence of the current sensor head. Variouscurrent and temperature sensors disclosed in this document can be usedto implement the sensor designs in FIGS. 3 and 4.

FIG. 5 shows another example of a magnetic/current sensor head based onthe design in FIG. 1 with additional features, where a Wollaston prismis used to replace the polarizer in FIG. 1 and a third optical detectoris used. The light beam of one of the state of polarization (SOP) isallowed to get to the mirror in the sensor head and be reflected backtowards the collimator. After passing through the 22.5 degree permanentFaraday rotator and the sensing Faraday material twice, the SOP of thereflected beam is around 45 degrees from the Wollaston's principle axisand being split by the prism into two beams of orthogonal SOPs with anangle. The collimators then focuses the two beams into two opticalfibers. Three optical detectors PD1, PD2 and PD3 are shown in thisparticular example. The photocurrents in PD1 and PD2 are used to detectthe powers of the two polarization components in the reflected probelight from the sensor head. PD3 is optional for monitoring the lightoutput from the low DOP light source. When the electrical gains for thedetection circuits are properly adjusted, the photovoltages from PD1 andPD2 can be expressed as:V ₁ =V ₀(1+sin 2θ),  (5)V ₂ =V ₀(1−sin 2θ)  (6)The polarization rotation angle caused by the magnetic field and by thetemperature can be expressed by the following equation:

$\begin{matrix}{{{\sin\mspace{11mu} 2\theta} = \frac{V_{1} - V_{2}}{V_{1} + V_{2}}},} & (7)\end{matrix}$where θ=α₁(T)M+α₂(T−T₀). It is clear that the rotation angle issensitive to the temperature.

FIG. 6 shows an example of a temperature sensor head based on the sensorhead design in FIG. 5 in which the Faraday sensing material in FIG. 5 isremoved and the magnetically shielded permanent Faraday rotator is usedfor sensing the temperature. The temperature T can be obtained by Eq.(7):T=T ₀+(V ₁ −V ₂)/[2α₂(V ₁ +V ₂)]  (8)when the T is obtained, the temperature induced rotation angle can beabstracted out and the true magnetic field induced rotation can beobtained.

The examples in FIGS. 5 and 6 represent a class of optical sensors thatoperate based on a use of two different optical polarizations by havinga polarization splitting device such as a Wollaston prism with two fiberlines.

A current sensor based on this design can include a light source thatproduces probe light; a first fiber line having a first fiber lineterminal coupled to receive the probe light from the light source and todirect the received probe light to a second fiber line terminal of thefirst fiber line; an optical current sensor head coupled to the secondfiber line terminal of the first fiber line to receive the probe lightand configured to include a polarization prism to separate the probelight into a first and second polarized input beams in differentpolarizations, a first Faraday material that is magnetically shieldedfrom an influence of any external magnetic field and located along anoptical path of the first polarized input beam, a second Faradaymaterial that is not magnetically shielded and placed in an optical pathof the first polarized input beam to sense a magnetic field produced bya current to be measured, an optical reflector downstream from the firstand second Faraday materials along an optical path of the firstpolarized input beam to reflect the first polarized input beam back tothe first and second Faraday materials and the polarization prism whichsplits the reflected light into first and second reflected light beamsin different polarizations, and a fiber coupler coupled to the secondfiber line terminal of the first fiber line to direct the firstreflected light beam into the first fiber line; a second fiber linecoupled to the fiber coupler in the optical current sensor head toreceive the second reflected light beam; an optical detection unitincluding a first optical detector coupled to the first fiber line toreceive at least a portion of the first reflected light beam in thefirst fiber line from the optical current sensor head that carriesinformation of the current to be measured, and a second optical detectorcoupled to the second fiber line to receive at least a portion of thesecond reflected light beam in the second fiber line from the opticalcurrent sensor head that carries information of the current to bemeasured; and a measurement module that receives detector outputs fromthe first and second optical detectors to obtain information of thecurrent.

A temperature sensor based on this design can include a light sourcethat produces probe light; a first fiber line having a first fiber lineterminal coupled to receive the probe light from the light source and todirect the received probe light to a second fiber line terminal of thefirst fiber line; an optical temperature sensor head coupled to thesecond fiber line terminal of the first fiber line to receive the probelight and configured to include a polarization prism to separate theprobe light into first and second polarized input beams of differentpolarizations, a Faraday material that is magnetically shielded from aninfluence of any external magnetic field and located along an opticalpath of the first polarized input beam and varies a Faraday rotationangle in response to a change in temperature at the Faraday material, anoptical reflector downstream from the Faraday material along an opticalpath of the first polarized input beam to reflect the first polarizedinput beam back to the Faraday material and the polarization prism whichsplits the reflected light into first and second reflected light beamsin different polarizations, and a fiber coupler coupled to the secondfiber line terminal of the first fiber line to direct the firstreflected light beam into the first fiber line; a second fiber linecoupled to the fiber coupler in the optical temperature sensor head toreceive the second reflected light beam; an optical detection unitincluding a first optical detector coupled to the first fiber line toreceive at least a portion of the first reflected light beam in thefirst fiber line from the optical temperature sensor head that carriesinformation of the temperature to be measured, and a second opticaldetector coupled to the second fiber line to receive at least a portionof the second reflected light beam in the second fiber line from theoptical temperature sensor head that carries information of thetemperature to be measured; and a measurement module that receivesdetector outputs from the first and second optical detectors to obtaininformation of the temperature.

The above optical sensor heads use probe light at a one opticalwavelength. In other implementations, probe light at two differentoptical wavelengths can be used for current sensing and temperaturesensing. FIGS. 7-11 show examples of such a design. Such a currentsensor based on optical sensing can include a first light source thatproduces a first probe light beam at a first optical wavelength; asecond light source that produces a second probe light beam at a secondoptical wavelength that is different from first optical wavelength; anoptical coupling device that receives and combines the first and secondprobe light beams into a combined probe beam with probe light at thefirst and second optical wavelengths; a fiber line coupled to receivethe combined probe light beam from the optical coupling device; and anoptical current sensor head coupled to the fiber line to receive thecombined probe light beam. The head can be configured to include aninput optical polarizer to filter the combined probe light beam toproduce a polarized input beam, a Faraday rotator to rotatepolarization, which is magnetically shielded from an external magneticfield and located along an optical path of the polarized input beam, adichroic filter located downstream from the Faraday rotator to receivelight from the Faraday rotator and configured to transmit light at thefirst optical wavelength and reflect light at the second opticalwavelength, a sensing Faraday material that is not magnetically shieldedand placed in an optical path of light transmitted by the dichroicfilter to sense a polarization rotation in light at the second opticalwavelength caused by a temperature change and a magnetic field of anelectric current at or near the sensing Faraday material, and an opticalreflector downstream from the sensing Faraday material to reflect thelight at the second optical wavelength back to the sensing Faradaymaterial, the dichroic filter, the Faraday rotator and the input opticalpolarizer to return to the fiber line along with reflected light at thesecond optical wavelength. The optical coupling device coupled to thefiber line receives reflected light from the optical current sensor headand splits the received light into a first reflected beam at the firstoptical wavelength and a second reflected beam at the second opticalwavelength. The device also includes a first optical detector coupled toreceive the first reflected beam that carries information of the currentto be measured and an influence of a temperature at the optical currentsensor head; a second optical detector coupled to receive the secondreflected beam that carries information of the temperature at thesensing Faraday material; and a measurement module that receivesdetector outputs from the first and second optical detectors to obtaininformation of the current to be measured and to compensate for aneffect to the current measurement by the temperature.

FIG. 7 is a first embodiment of a WDM (wavelength division multiplexing)based magnetic/current sensor with temperature monitoring andcompensation. Two low DOP light sources at λ1 and λ2 are combined with aWDM and then input into the optical head. A dichroic filter, whichpasses λ1 but reflects λ2, is inserted between the 22.5-degree permanentFaraday rotator and the sensing Faraday material. Since the reflected λ2light only experiences the influence of the 22.5-degree rotator and theassociated temperature induced polarization rotation θ₁=α₂(T−T₀), it canbe used to monitor the temperature of the optical head. On the otherhand, light of λ1 carries the polarization rotation information of boththe magnetic field and the temperature, 0=α₁(T)M+α₂(T−T₀). PD1 and PD2are used to measure the photocurrents of the λ1 and λ2 channelsrespectively. The monitoring PD1 m and PD2 m are optional and is notrequired for AC magnetic field measurement. For DC magnetic fieldmeasurements, they may be used to monitor the optical power drifts andcompensate for it in data processing. After θ₁ and T are obtained fromλ2 channel measurement, the magnetic field can be accurately obtainedas:M=[θ−θ₁(T)]/α₁(T)  (9)The relationship between α₁ and T of the Faraday sensing material can beobtained experimentally. In Eq. (9), θ₁ is assumed the same for both λ1and λ2. If θ₁ is different for probe light at λ1 and λ2, the ratio ofα₂(λ₁) and α₂(λ₂) can be measured and this measurement can be used tomodify θ₁(T) mathematically in the software.

FIG. 8 illustrates a second embodiment a WDM based magnetic/currentsensor with temperature monitoring and compensation. It differs fromFIG. 7 in that a single broad band low DOP light source is used toreplace the two light sources and to produce probe light at both opticalwavelengths λ1 and λ2, the. In addition, bandpass filters are placed infront of PDs to make sure only light of selected wavelength is detectedby the corresponding PD.

FIG. 9 illustrates a 3^(rd) embodiment of WDM based magnetic/currentsensor with temperature monitoring and compensation. The differencebetween FIG. 8 and FIG. 9 is how the two wavelength channels at λ1 andλ2 are separated at the detection side by using different opticalcoupler designs.

FIG. 10 shows a 4^(th) embodiment of a WDM based magnetic/current sensorwith temperature monitoring and compensation by using another way toseparate and monitor light from different WDM channels at λ1 and λ2.

FIG. 11 illustrates a 5^(th) embodiment of a WDM based magnetic/currentsensor with temperature monitoring and compensation. In thisconfiguration, a Wollaston prism is used to replace the polarizer in thesensing head, just as in FIG. 5. In addition, a WDM is used to separatethe two wavelength channels and direct them their correspondingphotodetectors PD1 and PD2. PDm is an optional monitoring photodetector,which is not required for AC magnetic field sensing. For DC magneticsensing, it can be used to monitor the power drift of the optical sourceand compensate the power drift either in software or in hardware byfeedback back control.

FIG. 12 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 1 andother examples described above. This sensing fiber can be a low linearbirefringence fiber or a spun fiber in some implementations. A Faradaymirror is used at the end of the sensing fiber to reflect light backtowards the light source. The advantage of this configuration, ascompared with that of FIG. 1, is that the sensing fiber can be made toencircle an electric current carrying conductor to eliminate theinfluence of magnetic fields outside of the enclosed loop.

In this embodiment, the sensing fiber that encircles the conductor tosense the magnetic field induced by the current flowing in the conductoris substantially free of undesired influence by other magnetic fieldsnot from the current under measurement. Compared with the previousembodiments, this embodiment has the advantages of insensitivity to theinterfering magnetic fields generated by conductors outside of the fiberloop and insensitivity to the installation of the fiber around theconductor. The other embodiments can be sensitive to the distance andorientation angles of the sensor head with respect to the conductor andis sensitive to the magnetic field generated by other current carryingconductors. Ideally, the sensing fiber should have no linearbirefringence and the so called spun fiber is preferred. The 90 degreeFaraday mirror is coupled to sensing fiber loop and is used tocompensate for the residual linear birefringence inside the sensingfiber.

For a given Verdet constant a fiber material, the sensing fiber can becoiled with multiple turns to increase the polarization rotation angle.Examples of implementations of this aspect can be found in an articleentitled “Advances in Optical Fiber-Based. Faraday Rotation Diagnostics”and published by White et al. of Lawrence Livermore National Laboratoryin the 17th IEEE International Pulsed Power Conference in July of 2009,providing examples for using a fiber loop for sensing a current based onthe Faraday effect (https://e-reports-ext.llnl.gov/pdf/376161.pdf).Another article by Day et al. entitled “Faraday rotation in coiled, monomode optical fibers: isolators, filters and magnetic sensors” andpublished in Optics Letters in May of 1982 provides additionalinformation on using a fiber coil for sensing a magnetic field based onthe Faraday rotation in the fiber(https://www.osapublishing.org/ol/abstract.cfm?uri=ol-7-5-238). Theabove articles are incorporated by reference as part of the disclosureof this patent document.

The sensor example in FIG. 12 is one implementation of a current sensorbased on optical sensing in a fiber coil. Such a sensor device caninclude a light source that produces probe light; a fiber line having afirst fiber line terminal coupled to receive the probe light from thelight source and to direct the received probe light to a second fiberline terminal of the fiber line; an optical polarizer coupled to thesecond fiber line terminal of the fiber line to receive the probe lightand configured to filter the probe light to produce a polarized inputbeam; a Faraday rotator coupled in an optical path of the polarizedinput beam to cause a Faraday rotation; a segment of sensing fibercoupled to receive the probe light from the Faraday rotator andconfigured to include a fiber coil that is wrapped around a segment of aconductor carrying a current to be measured to sense a magnetic fieldproduced by the current in the conductor; a Faraday reflector coupled toreceive light from the segment of sensing fiber and to reflect thereceived light back into the segment of sensing fiber to pass throughthe Faraday rotator to return to the fiber line; an optical detectionunit including an optical detector coupled to the fiber line to receiveat least a portion of the reflected light in the fiber line from theoptical current sensor head that carries information of the current tobe measured; and a measurement module that receives an detector outputfrom the optical detector to obtain information of the current.

Specifically, FIG. 13 illustrates an embodiment of a reflective magneticsensor based on using a length of fiber to replace the Faraday sensingmaterial in FIG. 5 with two fiber lines. Such a current sensor based onoptical sensing in a fiber coil can include a light source that producesprobe light; a first fiber line having a first fiber line terminalcoupled to receive the probe light from the light source and to directthe received probe light to a second fiber line terminal of the firstfiber line; a polarization prism coupled to the second fiber lineterminal of the first fiber line to receive the probe light and toseparate the probe light into a first and second polarized input beamsin different polarizations; a Faraday rotator located along an opticalpath of the first polarized input beam; a sensing fiber coil coupled toreceive the probe light from the Faraday rotator and configured to wraparound a segment of a conductor carrying a current to be measured tosense a magnetic field produced by the current in the conductor; aFaraday reflector coupled to receive light from the sensing fiber coilto reflect the received light back into the sensing fiber coil to passthrough the Faraday rotator and the polarization prism which splits thereflected light into first and second reflected light beams in differentpolarizations; a fiber coupler coupled to the second fiber line terminalof the first fiber line to direct the first reflected light beam intothe first fiber line; a second fiber line coupled to the fiber couplerin the optical current sensor head to receive the second reflected lightbeam; an optical detection unit including a first optical detectorcoupled to the first fiber line to receive at least a portion of thefirst reflected light beam in the first fiber line that carriesinformation of the current to be measured, and a second optical detectorcoupled to the second fiber line to receive at least a portion of thesecond reflected light beam in the second fiber line that carriesinformation of the current to be measured; and a measurement module thatreceives detector outputs from the first and second optical detectors toobtain information of the current.

Two different probe light wavelengths can be used in connection with asensing fiber coil. Such a sensor using a sensing fiber wrapped around aconductor for sensing the current carried by the conductor can include alight source that produces probe light at optical wavelengths includinga first optical wavelength a second optical wavelength that is differentfrom first optical wavelength; a fiber line coupled to receive the probelight from the light source; an optical polarizer coupled to the fiberline to filter the probe light from the fiber line to produce apolarized beam; a Faraday rotator coupled to receive the polarized beamto rotate polarization; a dichroic filter located downstream from theFaraday rotator to receive light from the Faraday rotator and configuredto transmit light at the first optical wavelength and reflect light atthe second optical wavelength; a sensing fiber coil placed in an opticalpath of light transmitted by the dichroic filter to receive thetransmitted light and configured to wrap around a segment of a conductorcarrying a current to be measured to sense a magnetic field produced bythe current in the conductor; a Faraday reflector coupled to receivelight at the second optical wavelength from the sensing fiber coil toreflect the received light back into the sensing fiber coil to passthrough the dichroic filter, the Faraday rotator and the polarizer toreturn to the fiber line along with reflected light at the secondoptical wavelength; a first optical detector coupled to receivereflected light from the fiber line at the first optical wavelength thatcarries information of the current to be measured and an influence of atemperature at the fiber coil; a second optical detector coupled toreceive reflected light from the fiber line at the second opticalwavelength that carries information of the temperature at the Faradayrotator; and a measurement module that receives detector outputs fromthe first and second optical detectors to obtain information of thecurrent to be measured and to compensate for an effect to the currentmeasurement by the temperature.

Specific examples for using two wavelengths and a dichroic filter areshown in FIGS. 14-19. FIG. 14 illustrates an embodiment of a reflectivemagnetic sensor using a length of fiber to replace the Faraday sensingmaterial in FIG. 7. FIG. 15 illustrates an embodiment of a reflectivemagnetic sensor using a length of fiber to replace the Faraday sensingmaterial in FIG. 8.

FIG. 16 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 9.FIG. 17 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 10.FIG. 18 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 11.FIG. 19 illustrates an embodiment of a reflective magnetic sensor usinga length of fiber to replace the Faraday sensing material in FIG. 11,with an additional WDM (WDM2) to separate the two wavelength channelsfor sending to PD3 and PD4 respectively.

FIG. 20 illustrates another embodiment of a reflective magnetic sensorusing a length of fiber as the sensing medium as an example of anothersensor design.

In this sensor design in FIG. 20, a current sensor based on opticalsensing in a fiber coil includes a light source that produces probelight; a first polarization splitting device coupled to receive theprobe light from the light source and produces light in a firstpolarization only as a first polarized light beam and light in a second,different polarization only as a second polarized light beam; a firstFaraday rotator placed to receive the first polarized light beam fromthe first polarization splitting device to produce an output light beamwith a rotated polarization; a second polarization splitting devicecoupled to receive the output light beam from the first Faraday rotatorto produce light in a third polarization only as a third polarized lightbeam and light in a fourth polarization different from the thirdpolarization as a fourth polarized light beam; a second Faraday rotatorplaced to receive the third polarized light beam from the secondpolarization splitting device to produce a second output light beam; afiber line coupled to receive the second output beam from the Faradayrotator and configured to include a sensing fiber coil that is wrappedaround a segment of a conductor carrying a current to be measured tosense a magnetic field produced by the current in the conductor; and aFaraday reflector coupled to receive light from the sensing fiber coiland to reflect the received light back into the sensing fiber coil toreturn to the second Faraday rotator which transmits the returned lightto the second polarization splitting device which splits the returnedlight into light in the third polarization and light in the fourthpolarization. This current sensor also includes a first optical detectorcoupled to receive and detect the returned light in the fourthpolarization from the second polarization splitting device which directsthe returned light in the third polarization to transmit through thefirst Faraday rotator to reach the first polarization splitting device;a second optical detector coupled to receive and detect the returnedlight in the second polarization from the first polarization splittingdevice; and a measurement module that receives an detector outputs fromthe first and second optical detectors to obtain information of thecurrent.

The example in FIG. 20 is a specific implementation, where the abovefirst polarization splitting device is represented by a PBS1, the firstoptical detector is represented by PD1, the second polarizationsplitting device is represented by a PBS2, and the second opticaldetector is represented by PD2. The first Faraday rotator is a 45-degreeFaraday rotator and the second Faraday rotator is a 22.5-degree Faradayrotator. Light from the light source is coupled to the optical head witha PM fiber or a SM fiber, and passes through an optional polarizer toclean up the polarization to be totally linear. The SOP of the light isaligned with the passing axis of PBS1 so that the light passes the PBS1with a minimum insertion loss. After PBS1, the SOP is rotated 45 degreesand be aligned to the passing axis of PBS2 so that the light passes PBS2with a minimum loss. After PBS2, light passes through a 22.5 degreeFaraday rotator and then is coupled into a sensing fiber with acollimator. At the other end of the fiber, the light is reflected by aFaraday mirror to pass through the 22.5 degree Faraday rotator thesecond time with a total 45 degrees SOP rotation when the magnetic fieldto be sensed is zero. The s component of the light is reflected out byPBS2 into PD1 and the p component passes through PBS2. The p componentis further rotated another 45 degrees by the 45 degree Faraday rotatorso that it is totally reflected by PBS1 into PD2. Again, thephotovoltages from the detected photocurrents can be expressed by Eqs.(5) and (6), assuming the electrical gains are properly adjusted, andthe polarization rotation angle can be obtained Eq. (7).V ₁ =V ₀(1−sin 2θ)  (10)V ₂ =V ₀(1+cos 2θ)  (11)

Because both sin and cos of the polarization rotation angle are present,any arbitrary large rotation angle can be obtained without ambiguity.Therefore, one may use a long sensing fiber to increase the detectionsensitivity without causing detection ambiguity.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination. Similarly, whileoperations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results.Moreover, the separation of various system components in the embodimentsdescribed in this patent document should not be understood as requiringsuch separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document and attachedappendices.

What is claimed is:
 1. A current sensor based on optical sensing,comprising: a light source that produces probe light of a low degree ofpolarization; a fiber line having a first fiber line terminal coupled toreceive the probe light from the light source and to direct the receivedprobe light to a second fiber line terminal of the fiber line; anoptical current sensor head coupled to the second fiber line terminal ofthe fiber line to receive the probe light and configured to include aninput optical polarizer to filter the probe light to produce a polarizedinput beam, a first Faraday material that is magnetically shielded froman influence of any external magnetic field and located along an opticalpath of the polarized input beam, a second Faraday material that is notmagnetically shielded and placed in an optical path of the polarizedinput beam to sense a magnetic field produced by a direct current or analternating current to be measured, and an optical reflector downstreamfrom the first and second Faraday materials along an optical path of thepolarized input beam to reflect the polarized input beam back to thefirst and second Faraday materials and the input optical polarizer toreturn to the fiber line; an optical detection unit including an opticaldetector coupled to the fiber line to receive at least a portion of thereflected light in the fiber line from the optical current sensor headthat carries information of the current to be measured and to generate adetector output signal carrying the information of the current and asecond optical detector coupled to the fiber line to receive and measureat least a portion of the probe light coming from the light sourcewithout passing through the second fiber line terminal of the fiber lineto provide a second detector signal as a measurement of the probe light;and a measurement module that is coupled to the optical detectionmodule, receives and processes the detector output signal from theoptical detector and the second detector signal as the measurement ofthe probe light from the second optical detector to obtain informationof the current.
 2. The current sensor as in claim 1, wherein themeasurement module is structured to be operable to apply the measurementof the probe light from the second optical detector to account for apower drift of the probe light of the low degree of polarization inobtaining the information of the current when the current is a directcurrent.
 3. The current sensor as in claim 1, wherein the opticalcurrent sensor head is free of an optical polarization element along anoptical path downstream from the first and second Faraday materials andupstream from the optical reflector.
 4. The current sensor as in claim1, wherein the measurement module includes an amplifier circuit toamplify the detector output from the optical detector, an analog todigital converter to convert an output of the amplifier circuit into adigital signal, and a microprocessor to process the digital signalrepresenting information of the current.
 5. A temperature sensor basedon optical sensing, comprising: a light source that produces probe lightof a low degree of polarization; a fiber line having a first fiber lineterminal coupled to receive the probe light from the light source and todirect the received probe light to a second fiber line terminal of thefiber line; an optical temperature sensor head coupled to the secondfiber line terminal of the fiber line to receive the probe light andconfigured to include an input optical polarizer to filter the probelight to produce a polarized input beam, a Faraday material that ismagnetically shielded from an influence of any external magnetic fieldand located along an optical path of the polarized input beam and variesa Faraday rotation angle in response to a change in temperature at theFaraday material, and an optical reflector downstream from the Faradaymaterial along an optical path of the polarized input beam to reflectthe polarized input beam back to the Faraday material and the inputoptical polarizer to return to the fiber line; an optical detectorcoupled to the fiber line to receive at least a portion of the reflectedlight in the fiber line from the optical temperature sensor head thatcarries information of the temperature to be measured to generate adetector output signal carrying the information of the temperature; asecond optical detector coupled to the fiber line to receive and measureat least a portion of the probe light coming from the light sourcewithout passing through the second fiber line terminal of the fiber lineto provide a second detector signal as a measurement of the probe light;and a measurement module coupled to receive the detector output signalfrom the optical detector and the second detector signal from the secondoptical detector and to obtain information of the temperature at thetemperature sensor head based on both the detector signal and the seconddetector signal.
 6. The temperature sensor as in claim 5, wherein themeasurement module includes an amplifier circuit to amplify the detectoroutput from the optical detector, an analog to digital converter toconvert an output of the amplifier circuit into a digital signal, and amicroprocessor to process the digital signal representing information ofthe temperature.
 7. An optical sensor system for measuring temperatureand an electric current based on optical sensing, comprising a basestation including a light source that produces probe light of a lowdegree of polarization, an optical coupler that splits the probe lightinto a first probe beam and a second probe beam; an optical sensor headthat is separate from the base station and includes a current sensorthat receives probe light from the first probe beam and a temperaturesensor that receives probe light from the second probe beam; a firstfiber line coupled between the base station and the current senor in theoptical sensor head to transmit the first probe beam to the currentsensor; and a second fiber line coupled between the base station and thetemperature sensor in the optical sensor head, wherein the currentsensor is configured to include an input optical polarizer to filter theprobe light to produce a polarized input beam, a first Faraday materialthat is magnetically shielded from an influence of any external magneticfield and located along an optical path of the polarized input beam, asecond Faraday material that is not magnetically shielded and placed inan optical path of the polarized input beam to sense a magnetic fieldproduced by a current to be measured, and an optical reflectordownstream from the first and second Faraday materials along an opticalpath of the polarized input beam to reflect the polarized input beamback to the first and second Faraday materials and the input opticalpolarizer to return to the first fiber line, wherein the base stationfurther includes a current optical detection unit including an opticaldetector coupled to the first fiber line to receive at least a portionof the reflected light in the first fiber line from the current sensorhead that carries information of the current to be measured to generatea detector output signal carrying the information of the current and asecond optical detector coupled to the first fiber line to receive andmeasure at least a portion of the probe light coming from the lightsource without passing through the first fiber line to reach the opticalsensor head to provide a second detector signal as a measurement of theprobe light, wherein the temperature sensor head is configured toinclude an input optical polarizer to filter the probe light to producea polarized input beam, a Faraday material that is magnetically shieldedfrom an influence of any external magnetic field and located along anoptical path of the polarized input beam and varies a Faraday rotationangle in response to a change in temperature at the Faraday material,and an optical reflector downstream from Faraday material along anoptical path of the polarized input beam to reflect the polarized inputbeam back to the Faraday material and the input optical polarizer toreturn to the second fiber line, and wherein the base station furtherincludes an optical detector coupled to the second fiber line to receiveat least a portion of the reflected light in the second fiber line fromthe temperature sensor head that carries information of the temperatureto be measured, and operates to process a detector output from theoptical detector coupled to the second fiber line to obtain informationon the temperature to be measured, and wherein the base station furtheroperates to process the detector output signal from the optical detectorand the second detector signal as the measurement of the probe lightfrom the second optical detector to obtain information of the current.